Understanding the cellular and molecular bases of behavior is one of the principal goals of contemporary neuroscience research. The fruit fly Drosophila melanogaster has proven to be an excellent model system for genetic and molecular studies of behavior and the nervous system. Recent advances, including techniques for cell-specific expression and the completion of the fly genome project, have underscored the utility of Drosophila for this type of functional analysis. In our continuing studies, we take advantage of the Drosophila system to better understand the circadian regulation of a behavior known as eclosion (the emergence or ecdysis of the Drosophila adult). The circadian regulation of this behavior is well understood in formal terms, and experimental studies have identified candidate cell types and gene products that are important modulatory elements of the clock output pathway controlling eclosion. Our previous studies identified a gene called lark, which encodes an RNA-binding protein that functions in the clock control of eclosion. Recent studies have documented circadian oscillations in LARK abundance, and shown that the protein is localized within peptidergic neurons (CCAP cells) that are essential for the physiological regulation of ecdysis (adult eclosion being the final ecdysis in the Drosophila life cycle). The CCAP cells contain a neuropeptide called Crustacean Cardioactive Peptide (CCAP), and CCAP release from these neurons drives ecdysis behavior. LARK exhibits a dramatic circadian oscillation in abundance within the CCAP neurons, and we hypothesize that the clock regulation of LARK within these peptidergic neurons is important for circadian control of eclosion. This continuation application is focused on the analysis of LARK within the CCAP neurons. The goals of the work are to: 1) analyze the regulation of LARK within the CCAP cell population, 2) ascertain whether the CCAP cells and CCAP neuropeptide are essential for the clock control of eclosion, 3) determine if LARK function within the CCAP cells is essential for rhythmicity, and 4) define the target mRNAs of LARK within the CCAP cell population. Such an analysis will provide significant information about the cellular and molecular signaling pathways governing the clock regulation of a specific behavior and contribute to a general framework for understanding the circadian control of behavior in mammalian and non-mammalian species.